In the first part of this series we compared the design of sleeve and ball bearing turbochargers and their pros/cons. Well, now it’s time to raise the curtain on an innovative Australian-built turbo bearing system...
Gold Coast based turbo specialist Mark ‘Mr Turbo’ Kitchen offers a range of turbochargers which employ an in-house developed silicon nitride bearing system. Mark says the biggest advantage of silicon nitride bearings is their ability to tolerate very high shaft speed and load - which means you can safely run more boost/power. The majority of Mr Turbo’s silicon nitride bearing systems are installed within a custom built carrier assembly and a CNC machined aluminium centre cartridge.
At the time of writing, Mr Turbo manufactures cartridges from relatively small T3/T4 size turbochargers all the way up to drag spec monsters capable of hundreds of kilowatts. The new cartridges are CNC machined from billet aluminium to accept off-the-shelf shafts, wheels and housings.
Mr Turbo manufactures a stainless steel bearing carrier tube which is inserted into the aluminium cartridge. On its inner surface, the carrier incorporates machined shoulders to locate a ball bearing race at each end while, on its outer surface, a grooved section along the top collects oil supplied from the engine. At each end of this groove is a small hole that lets oil drip onto the turbine shaft (to help remove some heat) and, more importantly, lubricate the ball bearings. Oil can escape through a large diameter outlet at the bottom of the cartridge.
Like an OE ball bearing turbocharger, the turbine shaft is supported by two bearings. However, a Mr Turbo turbocharger employs silicon nitride balls which are extremely hard and perfectly round, giving advantages in high rpm/load durability. The bearings are retained in NSK-manufactured inner and outer races and the cageless design contributes to further improved high rpm/load durability.
The inner bearing races spin at around turbine shaft speed while the outer races are able to turn relatively slowly. As seen here, a coil spring is fitted inside the bearing carrier to apply some preload and keep the bearings apart.
The complete bearing carrier assembly is slid into the custom machined centre cartridge and is locked into position by a bolt in one of the side water passages.
Note the large cooling passages around the bearing carrier – it’s important that any ball bearing turbo has significant cooling capacity because compared to a sleeve bearing, less heat is conducted away by the oil. In a Mr Turbo centre cartridge you can configure the coolant flow from left to right or vice versa.
Performance and Price
The biggest advantage of the Mr Turbo silicon nitride bearing turbo is its high rpm/load durability thanks to the harder and rounder ball bearings and cageless design. Mr Turbo has successfully pushed one of his silicon bearing turbochargers to over 300,000 rpm (calculated), which is significantly more than you’d achieve using a typical ball bearing unit. Depending on your intended application, you can either push more boost into your engine with greater safety or opt for what would normally be considered a ‘too small’ turbocharger – the extra high rpm/load durability will let you push it harder in the top-end while enjoying the driveability benefits of a small turbocharger.
Another advantage is the superior visual appeal of the Mr Turbo billet aluminium centre cartridge which can be anodised (as seen here). Perfect for show cars.
So what are the drawbacks of the new bearing system?
Well, the silicon nitride bearings shouldn’t be exposed to excessively high temperatures – the recommended maximum is 300 degrees Celsius. It’s for this reason that Mr Turbo emphasises the use of very large cooling passages within the centre cartridge. The other disadvantage is cost...
To purchase a complete Mr Turbo silicon nitride bearing turbocharger in a small T3/T4 size you’re looking at AUD$2300 – AUD$2400 (including housings and wheels). The largest versions will set you back just over AUD$5100. We suggest contacting Mr Turbo for prices on your specific requirements.
All complete turbo assemblies are supplied with a 12 months warranty on parts/manufacturing and long-term durability appears very good - the longest running example has so far lasted more than two years in a 22 psi boosted street car.
Verdict? A Mr Turbo silicon nitride bearing turbocharger is certainly worth considering if you’re searching for a new large capacity turbo – the claimed benefits seem reasonable and the extra cost is modest. It could well give you that all-important edge.
Mr Turbo +61 75596 1079